Provenance and tectonic setting of the Gudongjing Group in Beishan Orogen

Abstract

增生造山带中陆源碎屑岩物源区特征的研究可为解剖造山带结构甚至大陆地壳的形成和演化提供关键证据。北山造山带中部的古硐井群被认为是前寒武纪稳定沉积盖层，是北山造山带存在微陆块的重要依据。本文围绕古硐井群的物源区特征，进行了碎屑颗粒和重矿物统计、全岩主微量元素地球化学分析、碎屑锆石U-Pb年代学和Hf同位素测试。古硐井群碎屑岩的碎屑颗粒多呈棱角状，主要为石英、长石，同时含大量硅质岩碎屑和一定数量的火山岩碎屑；重矿物组成以褐铁矿、锆石、白钛石、尖晶石为主，角闪石、电气石、辉石次之，暗示物源区可能存在蛇绿岩、增生杂岩。全岩主量元素以高硅、高铝、富碱、低锰为特征，结合REE、Cr、Co、Sc和Th等惰性元素含量特征共同指示了长英质的物源区。最年轻的碎屑锆石年龄为443.9±13Ma，表明古硐井群最大沉积时限为晚奥陶世。碎屑锆石的年龄高度集中于470Ma附近，且该区间锆石ε_Hf（t）值多为正值，指示物源区存在大量新生地壳物质。本文推测古硐井群可能形成于增生楔楔顶盆地；研究结果支持北山造山带是古生代持续增生造山的产物这一认识。;The clastic sedimentary rocks in an accretionary orogenic belt may preserve key evidence for understanding the architecture of orogenic belts and even the crustal evolution. The clastic sedimentary rocks of the Gudongjing Group in the middle of the Beishan Orogen is considered to be a set of stable sedimentary caprocks of the Precambrian, and it is also an important basis for the existence of micro-continents in the Beishan Orogen. In this paper, statistics on clastic particle grain sizes and heavy mineral compositions, the whole rock geochemical composition analyses, detrital zircon U-Pb dating, and Hf isotope testing are carried out on the clastic rocks of the Gudongjing Group to reveal their prevenances and tectonic settings. The clastic components of the rocks are mainly composed of quartz, feldspar and rock fragments which are mainly siliceous rock fragments together with a certain amount of volcanic ones. Heavy mineral analysis reveals they mainly consist of limonite, zircon, ilmenite and spinel, followed by hornblende, tourmaline and pyroxene, which indicates the contribution of the accretionary complex materials and ophiolites to their provenance. The whole rock major element compositions are characterized by high contents of silicon, aluminum and alkali, while low in manganese. The contents of the inert elements, such as REE, Cr, Co, Sc, Th, etc., show characteristics of a felsic source area. The youngest detrital zircon age obtained in this study is 443.9±13Ma, indicating that the maximum deposition age of the Gudongjing Group is Late Ordovician. The age of detrital zircons is highly concentrated around 470Ma, and the corresponding zircon ε_Hf(t) values are mostly positive, indicating a large number of new crustal materials have developed in their provenance area. Based on the comparative analysis, it is suggested that these clastic rocks are possiblely formed in a slope basin on the top of an accretionary wedge. The results support the point that the Beishan orogenic belt is the product of a continuous accretion of a Paleozoic orogenic belt.